The present invention relates to a hollow plastic playing ball, and more particularly to a method of manufacturing a hollow polyvinyl chloride ball having an inflating valve so as to maintain an internal pressure for a long period.
Heretofore, there are various methods of manufacturing a hollow plastic playing ball by using polyvinyl chloride resin. Particularly, there is a well-known method of manufacturing an air-inflatable article having a valve therewith, in which said valve is firstly made of polyvinyl chloride resin containing a relatively large amount of a plasticizer and placed in a hollow metal mold and then a polyvinyl chloride resin paste for the article body is poured into the mold and thereafter the resulting body and the valve are integrally secured to each other. This valve has such a function that a leakage of air is prevented from the inside of the article after the air is filled in the inside of the ball through the valve by an air injection needle. However, the plasticizer contained in the valve penetrates into the body within some days after the molding, so that after the air injection needle is removed from the body, the size of the resulting pierced hole gradually increases and further the elasticity of the article is reduced. As a result, the air filled in the body gradually leaks out from the inside of the body through the pierced hole and the ball is deflated. Thus, the deflated article is worthless in the market and results in an economically serious loss.